Process for recovering CF4 and C2 F6 from vent gases of an aluminum production cell

ABSTRACT

A process for recovering at least one of CF 4  and C 2  F 6  from a vent gas from an aluminum electrolysis cell. The process includes the steps of: 
     (a) removing inorganic fluorides from a vent gas comprising inorganic fluorides and at least one of CF 4  and C 2  F 6  to obtain a purified vent gas; and 
     (b) contacting the purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF 4  and C 2  F 6 , and a permeate stream depleted in at least one of CF 4  and C 2  F 6 .

This application is a divisional of application Ser. No. 08/772,469,filed Dec. 23, 1996 now U.S. Pat. No. 5,814,127.

FIELD OF THE INVENTION

The present invention generally relates to a gas separation process. Theinvention particularly relates to a process for removing at least one ofCF₄ and C₂ F₆ from a gas stream using a membrane. The invention alsoparticularly relates to a method of making aluminum, which methodincludes removing at least one of CF₄ and C₂ F₆ from a gas stream usinga membrane.

BACKGROUND OF THE INVENTION

Currently, aluminum metal is commercially manufactured in two steps. Thefirst step involves extracting alumina (Al₂ O₃) from bauxite using aBayer process. The second step involves reducing the alumina which isdissolved in a mixture of molten cryolite (Na₃ AlF₆) and aluminumtrifluoride (AlF₃) in an electrolysis cell at about 950 to 960° C. usingthe Hall-Heroult process. In the second step, aluminum-containing ionsare reduced electrochemically to produce metallic aluminum at the metalcathode surface.

During normal electrolysis, carbon anodes are consumed asoxygen-containing ions react with the carbon anodes to form carbondioxide and aluminum metal according to the following equation:

    2Al.sub.2 O.sub.3 +3C→4Al+3CO.sub.2

However, at certain conditions, the electrolysis cell approaches ananode effect. This approach is characterized by the following events:

(1) The alumina concentration in the bulk of the electrolyte decreasesbelow 2% by weight;

(2) Higher concentrations of fluoride ions prevail near the anode as theconcentration of oxygen containing ions decreases;

(3) The anode polarization voltage increases significantly;

(4) The critical current density of the carbon anode is exceeded for thedischarge of only oxygen-containing anions; and

(5) F₂ is eventually discharged at the anode surface from thedecomposition of cryolite.

During anode effects, the fluorine discharged at the anode reacts withthe carbon to form CF₄ and C₂ F₆ according to the following equation:

    2Na.sub.3 AlF.sub.6 +2C→2Al+2NaF+CF.sub.4 +C.sub.2 F.sub.6

For a more detailed discussion of the anode effect, see Alton T.Tabereaux, Anode Effects, PFCs, Global Warming, and the AluminumIndustry, JOM, pp. 30-34 (November 1994).

For a typical electrolysis cell, the emission rate of CF₄ and C₂ F₆ perday is 0.25 kg. There are normally 100 to 200 cells per plant.Therefore, the daily emission of CF₄ and C₂ F₆ per plant is about 50 kg.

The emission of CF₄ and C₂ F₆ from aluminum plants has typically beenvented directly into the atmosphere. However, these gases, which are10,000 times more potent than CO₂, have recently been classified asglobal warming gases. Thus, with the signing of the United NationsFramework Convention on Climate Change which is aimed at reducing theemission of global warming gases, there is a significant need in theindustry for a way to minimize or eliminate the emission of these gasesinto the atmosphere.

Accordingly, it is an object of the present invention to address thisneed in the aluminum industry.

These and other objects of the invention will become apparent in lightof the following specification, and the appended drawings and claims.

SUMMARY OF THE INVENTION

The present invention relates to a process for recovering at least oneof CF₄ and C₂ F₆ from a vent gas from an aluminum electrolysis cell. Theprocess comprises the steps of:

(a) removing inorganic fluorides from a vent gas comprising inorganicfluorides and at least one of CF₄ and C₂ F₆ to obtain a purified ventgas; and

(b) contacting the purified vent gas with a membrane at conditionseffective to obtain a retentate stream rich in at least one of CF₄ andC₂ F₆, and a permeate stream depleted in at least one of CF₄ and C₂ F₆.

In another aspect, the present invention relates to a method of makingaluminum. The method includes the steps of:

(a) electrolytically reducing alumina dissolved in a mixture of moltencryolite and aluminum trifluoride in an electrolysis cell to producealuminum;

(b) withdrawing a vent gas comprising F₂, HF, and at least one of CF₄and C₂ F₆ from the electrolysis cell;

(c) contacting the vent gas with alumina at conditions effective toreact F₂ and HF with the alumina to produce aluminum trifluoride and agas stream comprising the at least one of CF₄ and C₂ F₆ ;

(d) recycling at least a portion of the aluminum trifluoride from step(c) to the electrolysis cell; and

(e) contacting the gas stream comprising the at least one of CF₄ and C₂F₆ with a membrane at conditions effective to obtain a retentate streamrich in at least one of CF₄ and C₂ F₆, and a permeate stream depleted inat least one of CF₄ and C₂ F₆.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a process flow diagram for recovering CF₄ and C₂ F₆according to the present invention.

FIG. 2 is a graph showing the relative permeabilities of N₂, CF₄, and C₂F₆ through a particular membrane at different temperatures.

FIG. 3 illustrates an engineering design of a membrane system that canbe employed in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its broadest aspect, the present invention relates to a process forremoving at least one of CF₄ and C₂ F₆ from a gas stream. Preferably,the present invention relates to a process for removing both CF₄ and C₂F₆ from a gas stream containing the same.

Prior to separation, the gas stream preferably contains from about 0.01to about 20% by volume of at least one of CF₄ and C₂ F₆. In addition tocontaining at least one of CF₄ and C₂ F₆, the gas stream can includeoxygen, carbon dioxide, and nitrogen as well as other gaseouscomponents.

The process according to the present invention comprises the step ofcontacting the gas stream with a membrane at conditions effective toobtain a retentate stream rich in at least one of CF₄ and C₂ F₆, and apermeate stream depleted in at least one of CF₄ and C₂ F₆. Preferably,the retentate stream is depleted in oxygen, carbon dioxide, and nitrogenwhile the permeate stream is rich in oxygen, carbon dioxide, andnitrogen. If both CF₄ and C₂ F₆ are present in the gas stream, then thecontacting step preferably produces a retentate stream rich in both CF₄and C₂ F₆, and a permeate stream depleted in both CF₄ and C₂ F₆.

As used in this specification and the claims, the term "rich" means thatthe concentration of a particular component in that stream is greaterthan the concentration of the same component in the feed stream.Likewise, the term "depleted" means that the concentration of aparticular component in that stream is less than the concentration ofthe same component in the feed stream.

The retentate stream preferably comprises from about 25 to about 100% byvolume of at least one of CF₄ and C₂ F₆. The permeate stream preferablycomprises from about 0 to about 0.01% by volume of at least one of CF₄and C₂ F₆.

In a preferred embodiment, the purity of the permeate and retentatestreams is improved by contacting the streams with additional membranesarranged in series. Such a process may be carried out according to themulti-step membrane separation system shown in FIG. 3. As shown in FIG.3, each of the permeate and retentate streams may be passed to anothermembrane contacting unit to increase the purity of the stream. Thenumber of contacting steps "m" and "n" may vary depending on the puritydesired. By using such a process, it is possible to obtain 100% recoveryof at least one of CF₄ and C₂ F₆ with a purity approaching 100%.

Any membrane can be used in the process according to the presentinvention so long as the membrane can selectively retain at least one ofCF₄ and C₂ F₆ while passing the other components in the gas streamthrough. The membrane should also be substantially non-reactive with thegaseous components to be separated.

Membranes useful in the invention are preferably glassy membranes suchas polymer membranes made preferably from polyimides; polyamides;polyamide-imides; polyesters polycarbonates; polysulfones;polyethersulfone; polyetherketone; alkyl substituted aromaticpolyesters; blends of polyethersulfone, aromatic polyimides, aromaticpolyamides, polyamides-imides, fluorinated aromatic polyimide,polyamide, and polyamide-imides; glassy polymeric membranes such asthose disclosed in U.S. Ser. No. 08/247,125 filed May 20, 1994, thecontent of which is hereby incorporated by reference; celluloseacetates; and blends thereof, copolymers thereof, substituted polymers(e.g. alkyl, aryl) thereof and the like.

Asymmetric membranes are prepared by the precipitation of polymersolutions in solvent-miscible nonsolvents. Such membranes are typifiedby a dense separating layer supported on an anisotropic substrate of agraded porosity and are generally prepared in one step. Examples of suchmembranes and their methods of manufacture are disclosed in U.S. Pat.Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; 4,512,893;5,085,676; and 4,717,394; all incorporated herein by reference. The '394and '676 patents disclose preparation of asymmetric separation membranesfrom selected polyimides. Particularly preferred membranes are polyimideasymmetric gas separation membranes as disclosed in the '676 patent.

In a pressure driven gas membrane separation process, one side of thegas separation membrane is contacted with a complex multicomponent gasmixture and certain of the gases of the mixture permeate through themembrane faster than the other gases. Gas separation membranes therebyallow some gases to permeate through them while serving as a barrier toother gases in a relative sense. The relative gas permeation ratethrough the membrane is a property of the membrane material compositionand its morphology. It has been suggested in the prior art that theintrinsic permeability of a polymer membrane is a combination of gasdiffusion through the membrane, controlled in part by the packing andmolecular free volume of the material, and gas solubility within thematerial. Selectivity is the ratio of the permeabilities of two gasesbeing separated by a material. It is also highly desirable to formdefect-free dense separating layers in order to retain high gasselectivity.

Composite gas separation membranes typically have a dense separatinglayer on a preformed microporous substrate. The separating layer and thesubstrate are usually different in composition. Composite gas separationmembranes have evolved to a structure of an ultrathin, dense separatinglayer supported on an anisotropic, microporous substrate. Thesecomposite membrane structures can be prepared by laminating a preformedultrathin dense separating layer on top of a preformed anisotropicsupport membrane. Examples of such membranes and their methods ofmanufacture are disclosed in U.S. Pat. Nos. 4,664,669; 4,689,267;4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922;2,970,106; 2,960,462; 4,713,292; 4,086,310; 4,132,824; 4,192,824;4,155,793; and 4,156,597; all incorporated herein by reference.

Alternatively, composite gas separation membranes may be prepared bymultistep fabrication processes, wherein first an anisotropic, poroussubstrate is formed, followed by contacting the substrate with amembrane-forming solution. Examples of such methods are described inU.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994; all incorporatedherein by reference.

U.S. Pat. No. 4,756,932 describes how composite hollow-fiber membranesmay also be prepared by co-extrusion of multiple polymer solutionlayers, followed by precipitation in a solvent-miscible nonsolvent.

According to one embodiment of the present invention, the membrane canbe post-treated with, or coated by, or co-extruded with, a fluorinatedor perfluorinated polymer layer in order to increase its ability towithstand harmful constituents in the gas mixture from which PFCs are tobe separate, at low levels or temporary contact with such components.

The hollow-fiber spinning process depends on many variables which mayaffect the morphology and properties of the hollow-fiber membrane. Thesevariables include the composition of the polymer solution employed toform the fiber, the composition of fluid injected into the bore of thehollow-fiber extrudate during spinning, the temperature of thespinneret, the coagulation medium employed to treat the hollow-fiberextrudate, the temperature of the coagulation medium, the rapidity ofcoagulation of the polymer, the rate of extrusion of the fiber, takeupspeed of the fiber onto the takeup roll, and the like.

The temperature of the gas mixture and/or the membrane during thecontacting step can vary from about -10° C. to about 100° C. Preferably,the temperature is between about 10° C. and 80° C. More preferably, thetemperature ranges from ambient, i.e., from about 20° C. to 25° C., toabout 60° C.

It is preferred, according to the present invention, to have a pressuredrop across the membrane of less than about 2,000 psig. More preferably,the pressure drop ranges from about 3 to about 200 psig. Even morepreferably, the pressure drop is about 20 to about 60 psig.

The requisite pressure drop across the membrane can be provided in oneof two ways. First, the feed gas stream can be compressed. Preferredcompressors are sealed and oil-free, such as the compressors sold underthe tradename POWEREX, available from the Powerex Harrison Company ofOhio. Second and more preferably, the pressure drop across the membranecan be established by lowering the pressure on the permeate side of themembrane. To create the lower pressure on the permeate side, a vacuumpump or any other suction device can be used.

The flowrate of the gas stream across the membrane can vary from about 0to about 10⁵ Nm³ /h per square meter of membrane available forseparation. Preferably, the flowrate is from about 10⁻⁴ to about 10 Nm³/h-m². More preferably, the flowrate is from about 0.1 to about 0.5 Nm³/h-m².

In a preferred aspect, the present invention relates to a process forrecovering at least one of CF₄ and C₂ F₆ from a vent gas from analuminum electrolysis cell. The vent gas comprises (1) gaseouscomponents such as O₂, CO₂, and N₂ ; (2) inorganic fluorides such as F₂,HF, and NaAlF₄ ; and (3) at least one of CF₄ and C₂ F₆. The vent gas canalso include some hydrocarbons and a large amount of particulates.

The first step in this process involves removing the inorganic fluoridesfrom the vent gas to produce a purified vent gas. The inorganicfluorides are preferably removed from the vent gas by using a causticscrubber. The scrubber can be wet or dry. Dry scrubbers are usuallyresin-type scrubbers or soda-lime, while some dry scrubbers comprisingcatalysts such as MnO₂ can also be used. Exemplary wet scrubbers thatcan be used in the present invention are described in the brochureentitled, "Selecting a CDO™ for Your Particular Application" fromDELATECH Corporation, which brochure is hereby incorporated byreference. When various harmful constituents have to be removed, it ispreferred to use a dry scrubber or scrubbers in series with a wetscrubber or scrubbers.

Preferably, upstream of the scrubber or scrubbers, one or more filtersare employed to remove the particulates from the vent gas. It ispreferred to use a filter having a pore size diameter of less than 20micrometers, and more preferably, less than 10 micrometers.

The second step in this process involves contacting the purified ventgas with a membrane at conditions effective to obtain a retentate streamrich in at least one of CF₄ and C₂ F₆, and a permeate stream depleted inat least one of CF₄ and C₂ F₆. This membrane separation step can becarried out as described above.

If there are remaining particulates in the purified vent gas before itis passed to the membrane separation unit, it is contemplated by thepresent invention to employ an additional filter or filters to removesuch particulates.

In another preferred aspect, the present invention relates to a methodof making aluminum. The method includes the steps of:

(a) electrolytically reducing alumina dissolved in a mixture of moltencryolite and aluminum trifluoride in an electrolysis cell to producealuminum;

(b) withdrawing a vent gas comprising F₂, HF, and at least one of CF₄and C₂ F₆ from the electrolysis cell;

(c) contacting the vent gas with alumina at conditions effective toreact F₂ and HF with the alumina to produce aluminum trifluoride and agas stream comprising the at least one of CF₄ and C₂ F₆ ;

(d) recycling at least a portion of the aluminum trifluoride from step(c) to the electrolysis cell; and

(e) contacting the gas stream comprising the at least one of CF₄ and C₂F₆ with a membrane at conditions effective to obtain a retentate streamrich in at least one of CF₄ and C₂ F₆, and a permeate stream depleted inat least one of CF₄ and C₂ F₆.

The general features of this method are illustrated in FIG. 1. Referringto FIG. 1, a stream 1 of alumina, cryolite, and aluminum trifluoride isfed into an aluminum electrolyzing cell 2 where molten aluminum isproduced by electrolytically reducing the alumina. The operatingconditions and equipment necessary to carry out this step are well knownto those skilled in the art.

The molten aluminum is then withdrawn from the electrolyzing cell 2through line 3. A vent gas stream 4 is also withdrawn from theelectrolyzing cell 2. The vent gas stream 4 comprises O₂, CO₂, N₂, F₂,HF, NaAlF₄, CF₄, C₂ F₆, and particulates. The vent gas stream 4 isoptionally passed to a filter 5 to remove the particulates therein.

A filtered vent gas 6 is optionally withdrawn from filter 5 and passedto a dry scrubbing zone 7. In the scrubbing zone 7, the filtered ventgas 6 is contacted with alumina introduced through line 8 at conditionseffective to remove inorganic fluorides such as F₂ and HF therefrom.These conditions for carrying out this scrubbing step are well known tothose skilled in the art. During the scrubbing step, the alumina reactswith F₂ and HF in the filtered vent gas 6 to form aluminum trifluorideaccording to the following equations:

    Al.sub.2 O.sub.3 +F.sub.2 →AlF.sub.3 +O.sub.2

    Al.sub.2 O.sub.3 +HF→AlF.sub.3 +H.sub.2 O

A stream 9 comprising the aluminum trifluoride is then withdrawn fromthe dry scrubbing zone 7 and recycled to stream 1. The dry scrubbingzone 7 also yields a purified vent gas stream 10 containing CF₄, C₂ F₆,O₂, CO₂, and N₂. Stream 10 is passed to an additional filter 11, ifnecessary, to remove any remaining particulates therein. The purifiedvent gas stream 10 is next passed to a membrane separation system 12such as the one described above and illustrated in FIG. 3 where aretentate stream 14 and a permeate stream 13 are produced. The permeatestream 13 comprises mostly of O₂, CO₂, and N₂, while the retentatestream 14 comprises mostly of CF₄ and C₂ F₆. The retentate stream 14comprising CF₄ and C₂ F₆ may be further purified on or off-site toproduce a CF₄ /C₂ F₆ stream suitable for use in the semiconductorindustry.

EXAMPLES

The following examples are provided to illustrate the present inventionand are not to be construed as a limitation thereof.

Example 1

A gas stream comprising CF₄, C₂ F₆, and N₂ was contacted with apolyimide, asymmetric composite hollow fiber membrane at varioustemperatures to measure the relative permeabilities of CF₄, C₂ F₆, andN₂. The gas stream had a constant flowrate rate of 170 sccm. Thepressure of the feed gas was kept constant at 3 bar. The permeabilitiesof each of these components are graphically shown in FIG. 2. As seenfrom FIG. 2, the selectivities of CF₄ /N₂ and C₂ F₆ /N₂ are on the orderof 300.

Based on the above selectivities, a computer simulation of a singlestage membrane separation unit was conducted. The concentration andpressure of the feed, permeate, and retentate as well as the CF₄ /C₂ F₆recovery are shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                                         CF.sub.4 /C.sub.2 F.sub.6                      Feed Permeate Retentate Recovery                                            ______________________________________                                        Flowrate 10     9.7         0.3                                                 (Nm.sub.3 /h)                                                                 [CF.sub.4 ] 0.1 1.2 × 10.sup.-3 3.9                                     (vol %)                                                                       [C.sub.2 F.sub.6 ] 0.1 1.5 × 10.sup.-3 3.9                              (vol %)                                                                       Pressure 20 0.2 19.7                                                          (bar)                                                                             98.7%                                                                   ______________________________________                                    

Example 2

The procedure of Example 1 was repeated using the feed concentration andpressure listed in Table 2 below. The simulated results are also listedin Table 2.

                  TABLE 2                                                         ______________________________________                                                                         CF.sub.4 /C.sub.2 F.sub.6                      Feed Permeate Retentate Recovery                                            ______________________________________                                        Flowrate 10     9.4         0.6                                                 (Nm.sub.3 /h)                                                                 [CF.sub.4 ] 1.0 0.04 16.5                                                     (vol %)                                                                       [C.sub.2 F.sub.6 ] 5.0 0.16 83.1                                              (vol %)                                                                       Pressure 30 0.2 29.7                                                          (bar)                                                                             96.9%                                                                   ______________________________________                                    

Example 3

For a typical smelting plant with 100 smelting pots, the flowrate of thevent gas is about 3 Nm³ /h. The same procedure of Example 1 was followedusing such a stream. The feed concentration and pressure as well as thesimulated results of such a stream are reported in Table 3 below.

                  TABLE 3                                                         ______________________________________                                                                         CF.sub.4 /C.sub.2 F.sub.6                      Feed Permeate Retentate Recovery                                            ______________________________________                                        Flowrate 3      2.6         0.4                                                 (Nm.sub.3 /h)                                                                 [CF.sub.4 ] 10.0 0.2 79.2                                                     (vol %)                                                                       [C.sub.2 F.sub.6 ] 1.0 0.02 7.9                                               (vol %)                                                                       Pressure 10 1.0 9.7                                                           (bar)                                                                             98.1%                                                                   ______________________________________                                    

Example 4

In an actual experiment, a feed stream comprising 1.14% CF₄ and C₂ F₆,and the balance N₂ was contacted with a polyimide, asymmetric compositehollow fiber membrane at room temperature. The flowrate, concentration,and pressure of the feed, permeate, and retentate streams along with thepercentage recovery of CF₄ and C₂ F₆ are reported in Table 4 below.

                  TABLE 4                                                         ______________________________________                                                                         CF.sub.4 /C.sub.2 F.sub.6                      Feed Permeate Retentate Recovery                                            ______________________________________                                        Flowrate 313.8   310        3.8                                                 (scfh)                                                                        [CF.sub.4 ] 0.45 <0.001 37.7                                                  (vol %)                                                                       [C.sub.2 F.sub.6 ] 0.69 0.003 57.9                                            (vol %)                                                                       Pressure 7.8 1.0 7.7                                                          (bar)                                                                             99.999%                                                                 ______________________________________                                    

While the invention has been described with preferred embodiments, it isto be understood that variations and modifications may be resorted to aswill be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and the scope ofthe claims appended hereto.

What is claimed is:
 1. A method of producing aluminum comprising thesteps of:(a) electrolytically reducing alumina dissolved in a mixture ofmolten cryolite and aluminum trifluoride in an electrolysis cell toproduce aluminum; (b) withdrawing a vent gas comprising F₂, HF, and atleast one of CF₄ and C₂ F₆ from said electrolysis cell; (c) contactingsaid vent gas with alumina at conditions effective to react F₂ and HFwith said alumina to produce aluminum trifluoride and a gas streamcomprising said at least one of CF₄ and C₂ F₆ ; (d) recycling at least aportion of said aluminum trifluoride from step (c) to said electrolysiscell; and (e) contacting said gas stream comprising said at least one ofCF₄ and C₂ F₆ with a membrane at conditions effective to obtain aretentate stream rich in at least one of CF₄ and C₂ F₆, and a permeatestream depleted in at least one of CF₄ and C₂ F₆.
 2. The methodaccording to claim 1, wherein said gas stream comprises both CF₄ and C₂F₆, and said retentate stream is rich in both CF₄ and C₂ F₆, and saidpermeate stream is depleted in both CF₄ and C₂ F₆.
 3. The methodaccording to claim 1, wherein said gas stream further comprises O₂, CO₂,and N₂.
 4. The method according to claim 3, wherein said retentatestream is depleted in O₂, CO₂, and N₂, and said permeate stream is richin O₂, CO₂, and N₂.
 5. The method according to claim 1, wherein said gasstream comprises from about 0.01 to about 20% by volume of at least oneof CF₄ and C₂ F₆.
 6. The method according to claim 5, wherein saidretentate stream comprises from about 20 to about 100% by volume of atleast one of CF₄ and C₂ F₆, and said permeate stream comprises fromabout 0 to about 0.01% by volume of at least one of CF₄ and C₂ F₆. 7.The method according to claim 1, wherein said conditions comprise atemperature between about 10 and about 80° C., a pressure drop betweenabout 3 and about 200 psig, and a flowrate rate between about 10⁻⁴ andabout 10 Nm³ /h-m².
 8. The method according to claim 1, wherein saidmembrane is selected from the group consisting of polyimides,polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones,polyethersulfone, polyetherketone, alkyl substituted aromaticpolyesters, and blends of polyethersulfone, aromatic polyimides,aromatic polyamides, polyamides-imides, fluorinated aromatic polyimide,polyamide, and polyamide-imides.